1. Field of the Invention
The present invention relates generally to data storage and more particularly to optical disk drive tracking and focus servo systems.
2. Description of the Prior Art
Magneto-optic data recording technology combines the erasability features of magnetic data storage systems with the high data storage capacity of optical systems. Current 5.25 inch magneto-optic disk can hold up to 600M bytes of information, 1000 or more times the amount of information that a similarly sized magnetic floppy diskette can store. Future media will hold considerably more data. Magneto-optic disks are also transportable and can be transferred between drives. Since the reading, writing and erasing operations are performed with light beams in combination with magnetic fields, they have long life, high reliability, and are relatively immune to physical wear.
The principles of magneto-optic technology are well known. Information is digitally stored at bit positions on a magneto-optic disk. The orientation of the magnetic field at each bit position can be switched between a first, or digital one state, in which its north pole is oriented upward, and a second, or digital zero state, in which the magnetic field is reversed and the north pole oriented downward. The orientation of the magnetic field at each bit position is selected by subjecting the bit position to a magnetic field of the appropriate polarity, and heating the bit position of the disk by the laser beam of write or erase intensity. The magnetic orientation of the bit position is "frozen" when the laser beam of write or erase intensity is removed and the disk cools and returns to room temperature after the laser is turned off.
The magnetic fields of all bit positions in an unwritten disk will generally be oriented with north poles down to represent digital zeros. When writing information, the bit positions will be subjected to a write magnetic bias field and heated by a high (write) intensity laser beam. The orientation of the magnetic fields at the written bit positions will reverse to north poles up. Bit positions are erased by subjecting them to an erase bias field of the opposite polarity, and again heating the bit position. The magnetic field orientation at the erased bit positions will then reverse and switch to north poles down.
Data is read from the optical disk using a low-power or read intensity laser beam. Because of the magneto-optic phenomenon known as the Kerr Effect, the polarization of a laser beam impinged upon the bit positions will be rotated as a function of the magnetic orientation of the bits. The polarization of laser beam portions reflected from bit positions on the optical disk is detected by opto-electronic detector circuitry. Signals from the detector circuitry are then processed to determine whether the bit position is representative of a digital one or zero.
Bit positions are aligned adjacent one another in an elongated servo track in a groove on the optical disk (media). The optical disk can include a single servo track which is spirally positioned on the disk, or a plurality of concentrically positioned servo tracks. The groove or grooves in which the servo tracks are positioned are separated by land portions. The laser beam used to read, write and erase data at the bit positions is focused onto the disk by an objective lens. Optical disk drives of this type typically include a focus servo system for driving the objective lens about a focus axis to keep the laser beam focused on the disk. A tracking servo system is used to drive the objective lens along a tracking axis perpendicular to the servo tracks and to maintain the laser beam centered over a desired servo track.
Tracking and focus servo systems for optical disk drives are generally known and illustrated, for example, in the Silvy et al. U.S. Pat. No. 4,700,056 which is specifically incorporated herein, by reference, for all that it teaches. After the laser beam has been modulated by the individual bit positions, it is reflected from the optical disk and impinges upon an optical detector typically having four or more separate detector sections arranged in a geometric pattern. Circuitry coupled to the optical detector produces both tracking and focus error signals. The focus error signal is generally sinusoidally shaped and has a magnitude and polarity which represent the distance and direction, respectively, from which the objective lens is displaced from proper focus. Similarly, the tracking error signal is a generally sinusoidal signal having a magnitude and polarity representative of the distance and direction by which the laser beam is offset from the center of a desired servo track.
The focus and tracking error signals are processed by the servo systems while operating in closed loop modes to generate focus and tracking drive signals. The focus and tracking drive signals are applied to respective actuators or motors and cause the objective lens to be driven to a position which minimizes the focus and tracking errors.
The focus and tracking servo systems have a limited response time in which they can respond. Servo errors or disturbances that generate required response times that are less than the response time of the servo system results in little or no closed-loop response by the system. This results from the limited ability of the mechanical system to make physical corrections. The error signal generated by the control servo system can be compensated by lag-lead and/or lead-lag filters for proper frequency characteristics before being applied to the actuator as a drive signal.
Typical processing steps performed on the focus and tracking error signals include the addition of an offset value to compensate for electrical, optical and/or mechanical characteristics of the drive and the multiplication factor for the error signal by a gain factor to control servo system response time.
A well known source of error in the focus servo loop is the introduction of a false focus error signal as a result of optical crosstalk. Errors in the focus servo loop caused by optical crosstalk degrade the overall performance of the disk drive. Optical crosstalk is a result of diffraction patterns formed from the different phase relationships of reflected light from the uneven surface of the grooved media. The reflected light and diffraction patterns impinge upon all sections of the optical detector. As a result of this distortion, or "optical crosstalk", changes in tracking position cause changes in the focus error signal.
The tracking error signal (TES) 1 illustrated in FIG. 1 to a first approximation, can be represented by the equation: EQU TES=A*sin (180*X/0.8) Eq. 1
In Equation 1, the variable A represents the peak amplitude of the tracking error signal 1. The number 0.8 represents half the track pitch (i.e., half the distance between centers of adjacent servo tracks). The variable X represents the difference, either plus or minus (+/-), between the center of the track and the location of the optical beam on the track.
The actual focus error signal (FES) is comprised of a "true" focus error component and a crosstalk induced component. The focus error signal can be approximated by the following equation: EQU FES=foc(y)+B sin[(180* X/0.8)+.phi.] Eq. 2
In Equation 2, the "true" focus error component is represented by foc(y) and a crosstalk induced component 7 illustrated in FIG. 2 is represented by the sin term. Like the corresponding values in Equation 1, the variable B represents the peak amplitude of the crosstalk induced component 7 of focus error signal 11, 0.8 represents half the pitch of the servo track, while X represents the difference, either plus or minus (+/-), between the center of the servo track and the location of the beam on the track. The variable .phi. represents the phase relationship between the tracking error signal 1 and the crosstalk induced component 7 of focus error signal 11.
By knowing the value of B and .phi. in Equation 2, it is possible to calculate an approximate value of the crosstalk induced component 7 of the focus error signal 11 as a function of tracking error signal 1. The crosstalk induced component 7 of the focus error signal 11 can be subtracted from the focus error signal 1 to yield an uncontaminated or "true" focus error signal (not shown).
However, predetermination of these parameters is virtually impossible. First, the magnitude of B and .phi. are very dependent on the particular media utilized, and vary from vendor to vendor. Typical variation for the value of B from media vendor to media vendor is 2-3 to 1. B and .phi. are also dependent to a large degree on the alignment accuracy of the particular optical head used in the drive and sensitive to particulate contaminants that can exist, or collect along the optical path of the optical head. It is also difficult to measure .phi.. Because of the variability of B and .phi. and the difficulty of accurately determining these values, methods which use B and .phi. to calculate the crosstalk component of the focus error signal and subtract this component from the focus error signal to obtain a "true" focus error signal are generally not commercially viable.
Other known techniques for reducing the effects of optical crosstalk include improving the quality of the optical systems and improving the optical focusing schemes. These techniques, however, have been impractical because the solution is vendor specific. These techniques are also expensive because of the adjustments and calibration necessary for each disk media used.
An optical focusing method which is said to provide good crosstalk performance under some circumstances is described in a paper entitled "New Focusing Method for Draw-Type Optical Head" by S. Arai, K. Hamada and K. Ogawa, presented at the topical meeting on Optical Data Storage, Oct. 15-17, 1985, in Washington, D.C. which is specifically incorporated herein, by reference, for all that it teaches. This focusing technique uses both a lens offset method and a gain difference method.
From the foregoing description, it is evident that known techniques for reducing focus crosstalk in the focus servo loop require a great deal of time and precision in either the optics manufacture or in calibrating and aligning the detectors. These techniques contribute to the overall expense of the drive. Since the phase shift of reflected light depends among other things on the optical characteristics of the disk, and in particular, variations in track groove depths, previous methods of reducing optical crosstalk have required the use of the particular type of disk media for which the optical disk drive was calibrated. Furthermore, there are other factors which contribute to optical crosstalk such as temperature, aging and contamination on the disk surface which are not compensated for by the previous methods of optical crosstalk reduction.
It is evident that there is a continuing need for improved methods for attenuating optical crosstalk which introduces focus error in the focus servo loop. In particular, what is needed is an efficient and accurate and adaptive technique for eliminating the focus crosstalk component of the focus error signal for any particular media which is in the optical drive.